This application discloses an invention that is related, generally and in various embodiments, to a system and method for limiting input voltage to a power supply system having regeneration capability.
Power delivery systems such as AC motor drives are generally available as either a fully regenerative system or as a non-regenerative system. For non-regenerative systems, input power flows from an input power source to the load, and power produced by the load is blocked from flowing back to the input power source. In fully regenerative systems, input power flows from an input power source to the load, and the power produced by the load may flow back to the input power source.
It is known in the art for such fully regenerative systems to comprise an active front end to control regenerative current. However, active front ends may tend to cause some level of distortion of the incoming AC power. To reduce the distortion and smooth the input current on each leg of the AC input, it is also known to utilize line reactors on each leg of the AC input. However, the line reactors tend to be a relatively bulky and expensive solution to the distortion problem.
For many applications, only a portion of the power produced by the load is required for purposes of braking, etc. For such applications, a regenerative system generally provides much more regeneration capability than is required. Accordingly, most regenerative systems are not very cost-effective for applications which require limited regeneration capability.
To realize a drive with limited regeneration capability, it is known to configure a drive with a combination of regenerative and non-regenerative cells. For example, U.S. patent application Ser. No. 11/419,064 (the '064 application) discloses such a drive. The drive disclosed by the '064 application may be configured to have varying levels of regeneration capability by varying the number of regenerative versus non-regenerative cells in the drive, and each regenerative power cell may comprise an active front end that comprises switching devices controlled by pulse width modulation. Although the drive disclosed in the '064 application provides a cost-effective solution for applications requiring limited regeneration capability, any use of line reactors therewith to reduce any distortions caused by the active front end unnecessary adds to the size and cost of the system.
U.S. patent application Ser. No. 11/540,232 (the '232 application) discloses a method for smoothing input current to a power delivery system having regeneration capability, and utilization of the method eliminates the need for the use of line reactors at the inputs of the regenerative power cells. When the method is being utilized, the power delivery system may be considered to be operating in a six step mode.
FIG. 1 illustrates various embodiments of a regenerative four quadrant power cell that may be utilized with the method described in the '232 application. The front end of the regenerative power cell includes six insulated gate bipolar transistors (Qap, Qbp, Qcp, Qan, Qbn and Qcn) and six diodes. When the drive is motoring and power is flowing from cell input to cell output, the free-wheeling diodes of the front end are conducting current. When the drive is regenerating and the power is flowing from cell output to cell input, the insulated gate bipolar transistors are conducting current. When the front end of the regenerative power cell is switching in the six step mode, cell input reactors are not needed due to the nature of the method. When the front end insulated gate bipolar transistors are switching in the six step mode, they are turning on or off the way diodes naturally do in a three phase diode rectifier of a non-regenerative cell.
FIG. 2 illustrates various embodiments of a switching sequence for the front end of the regenerative power cell of FIG. 1. As shown in FIG. 2, in every ⅙ fundamental cycle (60 degrees), a pair of insulate gate bipolar transistors, one from the upper bridge and the other from the lower bridge, are turned on, which results in the highest line-to-line voltage being applied to the DC bus. Therefore, a fundamental cycle (360 degrees) can be regarded as being divided into 6 windows, with each window being 60 degrees and having a different pair of insulated gate bipolar transistors turned on.
In the six step mode, the DC bus voltage of the power cell is not controlled. Therefore, if the line impedance is high and the drive is regenerating heavily (the output torque is high), at near rated speed where the primary regeneration current is the highest, the drive input voltage may increase to a point where one or more of the power cells trips on a DC bus over voltage fault.